[MIPS] Add seccomp bpf support

sandbox/linux/seccomp-bpf/syscall_iterator.cc

Index: sandbox/linux/seccomp-bpf/syscall_iterator.cc
diff --git a/sandbox/linux/seccomp-bpf/syscall_iterator.cc b/sandbox/linux/seccomp-bpf/syscall_iterator.cc
index 89cc1cb473cc911cd0c1254129624f13d422174c..3e624fc4115155fbfda676bf982ab600714d28e2 100644
--- a/sandbox/linux/seccomp-bpf/syscall_iterator.cc
+++ b/sandbox/linux/seccomp-bpf/syscall_iterator.cc
@@ -16,9 +16,15 @@ uint32_t SyscallIterator::Next() {
 
   uint32_t val;
   do {
+#if defined(__mips__) && (_MIPS_SIM == _MIPS_SIM_ABI32)
+    // |num_| has been initialized to 4000, which we assume is also MIN_SYSCALL.
+    // This is true for Mips O32 ABI.
+    COMPILE_ASSERT(MIN_SYSCALL == __NR_Linux, min_syscall_should_be_4000);

[jln (very slow on Chromium), 2014/05/02 20:42:04]

The syscall iterator should always start at 0, this is why this compile assert
was here.
So, in the constructor, keep initializing num_ to 0.

Here, do something similar to the version in this CL:
https://chromiumcodereview.appspot.com/11096012/diff/1/sandbox/linux/seccomp-...

i.e. if MIN_SYSCALL is not 0, the iterator should first return MIN_SYSCALL -1
before continuing to iterate.

[nedeljko, 2014/05/07 15:40:05]

I initialized num_ to 0 in constructor in new patch.
However this assert checks value of MIN_SYSCALL and on MIPS this value is not
zero.

+#else
     // |num_| has been initialized to 0, which we assume is also MIN_SYSCALL.
     // This true for supported architectures (Intel and ARM EABI).
     COMPILE_ASSERT(MIN_SYSCALL == 0u, min_syscall_should_always_be_zero);
+#endif
     val = num_;
 
     // First we iterate up to MAX_PUBLIC_SYSCALL, which is equal to MAX_SYSCALL